The microwave region of the electromagnetic spectrum finds widespread use in various fields of technology. Exemplary applications include wireless communication systems, such as mobile communication and satellite communication systems, as well as navigation and radar technology. The growing number of microwave applications increases the possibility of interference occurring within a system or between different systems. Therefore, the microwave region is divided into a plurality of distinct frequency bands. To ensure, that a particular device only communicates within the frequency band assigned to this device, microwave filters are utilized to perform band-pass and band reject functions during transmission and/or reception. Accordingly, the filters are used to separate the different frequency bands and to discriminate between wanted and unwanted signal frequencies so that the quality of the received and of the transmitted signals is largely governed by the characteristics of the filters. Commonly, the filters have to provide for a small bandwidth and a high filter quality.
For example, in communications networks based on cellular technology, such as the widely used GSM system, the coverage area is divided into a plurality of distinct cells. Each cell is assigned to a base station which comprises a transceiver that has to communicate simultaneously with a plurality of mobile devices located within its cell. This communication has to be handled with minimal interference. For example, base stations and mobile devices communicating based on GSM in the 900 MHz band must be protected from interference signals caused by communications based on GSM in the 1800 MHz band or UMTS. Moreover, the base stations and mobile devices should not transmit outside their designated frequency band. Therefore, the frequency range utilized for the communications signals associated with the cells is separated from adjacent frequencies by the use of microwave filters in the base station as well as in the mobile devices. Further, because GSM base stations transmit and receive simultaneously, the same microwave filters are also used to divide the frequency range into a first frequency band, that is used by the base station to transmit signals to the mobile devices (downlink), and a second frequency band, that is used by the mobile devices to transmit signals to the base station (uplink), in order to isolate the transmitter from the receiver. The filters must have a high attenuation outside their pass-band and a low pass-band insertion loss in order to satisfy efficiency requirements and to preserve system sensitivity. Thus, such communication systems require an extremely high frequency selectivity in both the base stations and the mobile devices which often approaches the theoretical limit.
Due to the ever decreasing size of the system components of such communication systems, it is further required that the filters are constructed as small and compact as possible. One particular type of resonator device suitable for achieving a particularly compact construction is resonator devices of the laminated type. These resonator devices comprise a plurality of dielectric layers in a stacked arrangement and electrically conductive layers provided on the outer surfaces of the stacked arrangement and/or sandwiched between the dielectric layers. These dielectric and electrically conductive layers are provided as a laminate. In known resonator devices of this type, an electrically conductive layer is disposed, in the stacking direction of the stacked arrangement, between two grounded electrically conductive layers. This structure, comprising two outer electrically conductive layers and an intermediate electrically conductive layer extending between the outer electrically conductive layers and separated from them by one or more layers of dielectric material, constitutes a strip-line transmission line. In such strip-line transmission lines electromagnetic waves travel in the direction of extension of the electrically conductive layers and, thus, in the direction of extension of the dielectric layers of the laminate that can be regarded as the “horizontal direction” of the resonator device. Strip-line transmission lines can be regarded as (pseudo) coaxial transmission lines with the outer electrically conductive layers functioning as and constituting the outer conductor and the intermediate electrically conductive layer functioning as and constituting the inner conductor, i.e. strip-line transmission lines show (pseudo) coaxial characteristics. This means that such a strip-line transmission line has the same characteristic impedance as a coaxial transmission line having a particular inner diameter and a particular outer diameter, so that the strip-line transmission line, like any (pseudo) coaxial transmission line, can be regarded as being equivalent to a coaxial transmission line having an effective inner diameter and an effective outer diameter.
Generally, a (pseudo) coaxial transmission is any multi-conductor transmission line that comprises one or more electrically connected first conductors and one or more electrically connected second conductors that essentially coextend, separated by a dielectric material, along the transmission line, wherein the first electrical conductor(s) function as the inner conductor and the second electrical conductor(s) function as the outer conductor. In the same manner as the above strip-line transmission lines, such transmission lines have (pseudo) coaxial characteristics and have the same characteristic impedance as a coaxial transmission line having a particular inner diameter and a particular outer diameter, so that they can be regarded as being equivalent to a coaxial transmission line having an effective inner diameter and an effective outer diameter. It should be noted that a two-wire transmission line already constitutes a (pseudo) coaxial transmission line with the grounded wire functioning as the outer conductor and the signal carrying wire functioning as the inner conductor. Thus, in the present application a (pseudo) coaxial transmission line is a multi-conductor transmission line constructed in the above-described manner. Preferably, these multi-conductor transmission lines are constructed such that one second electrical conductor at least partly or completely surrounds the first electrical conductor(s) or that there are two or more second electrical conductors that are spaced around the first electrical conductor(s). Usually, as in a regular coaxial transmission line, the first electrical conductor(s) would be electrically isolated from the second electrical conductor(s). However, as will be described later-on, the (pseudo) coaxial transmission lines of the present application are short-circuited at one end.
Independent of the particular implementation of a resonator device, one common resonator type is the quarter-wave-resonator in which a piece of an arbitrary type of transmission line having a length of one quarter wavelength is short circuited at one end and driven open at the other end to achieve the desired resonance. In reality, such resonators are always shorter than one quarter wavelength, because the open circuit cannot be ideally realized due to fringe fields that are always present at the open end which therefore acts as a capacitor. By increasing the capacitance at the open end, an additional length reduction of the transmission line can be achieved. To further shorten the transmission line, quarter wave resonators have been constructed having a transmission line with two distinct sections that have different characteristic impedances Z1 and Z2 with the low impedance section being provided at the short-circuited end and the high impedance section being provided at the open circuited end. These resonators are commonly referred to as stepped-impedance-resonators (SIR). In a simplified equivalent LC resonator model, the short-circuited high impedance section 1 can be regarded as an inductor with inductance
      L    =                            Z          1                ·                  tan          ⁡                      (                                          β                1                            ·                              l                1                                      )                              ω        ,
where β1 is the phase term of the propagation constant (β1=2π/λ1, with λ1 the wavelength for the given transmission line) and l1 the length of the section and ω the angular frequency, and the open circuited low impedance section 2 can be regarded as a capacitor with capacitance
      C    =                            Y          2                ·                  tan          ⁡                      (                                          β                2                            ·                              l                2                                      )                              ω        ,
where Y2=1/Z2 is the characteristic admittance, β2 the phase term of the propagation constant and l2 the length of the section. Thus, to achieve sufficiently large values of L, the impedance Z1 of that section has to be chosen to be large, and to achieve sufficiently large values of C, the admittance Y2 has to be chosen to be large.
For the above-mentioned resonator devices of the laminated type, which are produced by using e.g. the well established low temperature co-fired ceramics (LTCC) process, the strip-lines are realized by e.g. printing thin conductor layers on dielectric layer substrates followed by laminating and sintering the layers. Currently, for the common LTCC processes the thickness of the conductor layers is limited to 10 to 20 μm. Thus, in order to control the characteristic impedance of the strip-line, generally the width of the intermediate conductor layer has to be varied. Presently, for common LTCC processes the minimum width of the intermediate conductor layer is limited to around 80 to 100 μm. To realize the high impedance section, the width of the conductor should be small (i.e. minimum, e.g. 100 μm), and to realize the low impedance section, the width must be set to a larger value (e.g. 600 μm). Such resonator devices are e.g. disclosed in U.S. Pat. No. 5,719,539.
The exact values of the impedances of the strip-line section can be determined by numerical computation as well as by accurate equations or approximations. The influence of the dimensions of the strip-line arrangement can also be assessed by regarding it as a coaxial transmission line with effective inner diameter Di and effective outer diameter Do. The effective diameters are related to the exact geometry of the structure. For example a larger width of strip-line leads to larger Di, and a smaller height of the overall laminated resonator device and, thus, a smaller distance between the two outer conductive layers leads to a smaller Do. The characteristic impedance of a coaxial transmission line is given by
      Z    =                            Z          i                ·                  ln          ⁡                      (                                          D                0                            /                              D                i                                      )                                      2        ⁢                                  ⁢        π              ,
where Zi is the intrinsic impedance of the dielectric material given by Zi=√{square root over (μ/∈)}. As can be taken from this approximation, the characteristic impedance is only a function of the ratio of outer to inner diameters. Depending on the application, the height of current LTCC filters is around 850 μm, but also low profile filters with a height of 400 μm are of interest for more compact designs leading to even lower characteristic impedance values. Furthermore, since the volume of the resonator device is decreased, the stored energy is limited.
To realize a band pass filter with low insertion loss, the quality factor of the corresponding resonators should be as large as possible. The quality factor is determined by the ratio of stored energy to losses in the resonator, and there are mainly dielectric losses and conductor losses of the conductor which contribute to the quality factor Q in accordance with the equation:
      1    Q    =            1              Q        d              +                  1                  Q          c                    .      
Usually, for a suitably chosen dielectric material dielectric losses are lower than conductor losses, i.e. Qc is limiting the overall quality factor. If the volume of the resonator is increased, the dielectric quality factor Qd stays the same, but the conductive quality factor Qc may increase due to an increase of the ratio of volume vs. surface of the structure. Therefore, a large volume of the resonator is desirable for increasing the overall quality factor.
The conductive quality factor is further influenced by the current distribution within the electrically conductive layers of the strip-line arrangement. Due to the large aspect-ratio (width to height ratio) of the inner strip-line conductor, the current is generally concentrated at the edges of the conductor. However, in order to improve the quality factor the current should be distributed more homogeneously over the surface of the conductor. In U.S. Pat. No. 6,965,284 it is suggested to dispose a dielectric material with higher dielectric constant than the surrounding dielectric material centrally above and below the inner strip-line conductor in order to equalize the current distribution. Another approach is suggested in U.S. Pat. No. 6,020,798 and U.S. Pat. No. 6,346,866 in which the thickness of the inner strip-line conductor is increased by burying the inner strip-line conductor in a dielectric layer.
However, in the prior art devices of the laminated type including strip-line transmission lines extending in the direction of extension of the dielectric layers or horizontal direction, the quality factor achievable is still limited, and known measures for improving the quality factor add complexity and costs to the manufacturing processes.
U.S. Pat. No. 5,945,892 discloses an LC resonating device of laminated type in which a first capacitor electrode layer, a second capacitor electrode layer and a ground electrode layer are disposed, in this order, within a laminate including a plurality of dielectric layers. The electrically conductive layers are separated from each other by at least one of the dielectric layers. The first capacitor electrode is electrically connected to an external ground electrode provided as a layer on a lateral side surface of the laminate. Two electrically conductive via holes extend in the stacking direction of the laminate—which may be regarded as the “vertical direction” of the resonating device—between and electrically interconnect the ground electrode layer and the first capacitor electrode layer. Further, an electrically conductive via hole extends in the stacking direction of the laminate between and electrically interconnects the ground electrode and the second capacitor electrode. The via holes are arranged such that the latter via hole extending between the ground electrode and the second capacitor electrode constitutes an inductor conductive body, i.e. it is a lumped element inductor at which the inductance of the resonator is concentrated. However, this reference does not disclose a resonator device including a resonator comprising a transmission line, in particular a (pseudo) coaxial transmission line.